NAME
recv - receive a message from a connected socket
SYNOPSIS
#include <sys/socket.h>
ssize_t recv(int socket, void *buffer, size_t length, int flags);
DESCRIPTION
The recv() function shall receive a message from a connection-mode or
connectionless-mode socket. It is normally used with connected sockets
because it does not permit the application to retrieve the source
address of received data.
The recv() function takes the following arguments:
socket Specifies the socket file descriptor.
buffer Points to a buffer where the message should be stored.
length Specifies the length in bytes of the buffer pointed to by the
buffer argument.
flags Specifies the type of message reception. Values of this argument
are formed by logically OR’ing zero or more of the following
values:
MSG_PEEK
Peeks at an incoming message. The data is treated as unread and
the next recv() or similar function shall still return this
data.
MSG_OOB
Requests out-of-band data. The significance and semantics of
out-of-band data are protocol-specific.
MSG_WAITALL
On SOCK_STREAM sockets this requests that the function block
until the full amount of data can be returned. The function may
return the smaller amount of data if the socket is a message-
based socket, if a signal is caught, if the connection is
terminated, if MSG_PEEK was specified, or if an error is pending
for the socket.
The recv() function shall return the length of the message written to
the buffer pointed to by the buffer argument. For message-based
sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the entire message
shall be read in a single operation. If a message is too long to fit
in the supplied buffer, and MSG_PEEK is not set in the flags argument,
the excess bytes shall be discarded. For stream-based sockets, such as
SOCK_STREAM, message boundaries shall be ignored. In this case, data
shall be returned to the user as soon as it becomes available, and no
data shall be discarded.
If the MSG_WAITALL flag is not set, data shall be returned only up to
the end of the first message.
If no messages are available at the socket and O_NONBLOCK is not set on
the socket’s file descriptor, recv() shall block until a message
arrives. If no messages are available at the socket and O_NONBLOCK is
set on the socket’s file descriptor, recv() shall fail and set errno to
[EAGAIN] or [EWOULDBLOCK].
RETURN VALUE
Upon successful completion, recv() shall return the length of the
message in bytes. If no messages are available to be received and the
peer has performed an orderly shutdown, recv() shall return 0.
Otherwise, -1 shall be returned and errno set to indicate the error.
ERRORS
The recv() function shall fail if:
EAGAIN or EWOULDBLOCK
The socket’s file descriptor is marked O_NONBLOCK and no data is
waiting to be received; or MSG_OOB is set and no out-of-band
data is available and either the socket’s file descriptor is
marked O_NONBLOCK or the socket does not support blocking to
await out-of-band data.
EBADF The socket argument is not a valid file descriptor.
ECONNRESET
A connection was forcibly closed by a peer.
EINTR The recv() function was interrupted by a signal that was caught,
before any data was available.
EINVAL The MSG_OOB flag is set and no out-of-band data is available.
ENOTCONN
A receive is attempted on a connection-mode socket that is not
connected.
ENOTSOCK
The socket argument does not refer to a socket.
EOPNOTSUPP
The specified flags are not supported for this socket type or
protocol.
ETIMEDOUT
The connection timed out during connection establishment, or due
to a transmission timeout on active connection.
The recv() function may fail if:
EIO An I/O error occurred while reading from or writing to the file
system.
ENOBUFS
Insufficient resources were available in the system to perform
the operation.
ENOMEM Insufficient memory was available to fulfill the request.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
The recv() function is equivalent to recvfrom() with a zero address_len
argument, and to read() if no flags are used.
The select() and poll() functions can be used to determine when data is
available to be received.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
poll() , read() , recvmsg() , recvfrom() , select() , send() ,
sendmsg() , sendto() , shutdown() , socket() , write() , the Base
Definitions volume of IEEE Std 1003.1-2001, <sys/socket.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .